1. Field of the Invention
The present invention relates to a brushless motor and a magnetic recording-reproducing apparatus using the same.
2. Description of the Related Art
Hitherto, known brushless motors have been used as spindle motors for rotating, for example, floppy disks (FD) in magnetic recording-reproducing apparatuses. One brushless motor comprises a stator including a plurality of substrates radially protruding from a central shaft and coils wound on each of the plurality of substrates, and a rotor including a cover for covering the stator, the cover having a plurality of magnetic poles annularly arranged opposing the tip-end faces of the plurality of substrates.
A known brushless motor used for a magnetic recording-reproducing apparatus is described as follows in conjunction with drawings.
FIG. 6 is a sectional view showing a known brushless motor. FIG. 7 is an expanded sectional view showing the section VII of the brushless motor shown in FIG. 6. FIG. 8 is a plan view showing a rotor of the known brushless motor. FIG. 9 is a plan view showing a stator and a reinforcing plate of the known brushless motor.
In FIGS. 6 to 9, the known brushless motor of generally 3 mm in thickness, including a stator 10, a rotor 20, and a reinforcing plate 30, is driven by three-phase alternating current.
The stator 10 includes the following parts or components: a rotatable center shaft 11; a bearing unit 12 of an oil-retaining metal receiving the center shaft 11; a bearing case 13 holding the bearing unit 12 together with the center shaft 11; twenty-four substrates 14-1 to 14-24 radially protruding from the peripheral face of the bearing case 13 with a constant angular distance of 15 degrees (360 degrees/24=15 degrees) from each other; coils 15-1 to 15-24, each coil being wound on each of yokes 14a-1 to 14a-24 of the substrates 14-1 to 14-24; and three position-detecting Hall elements 16, each element being disposed at a predetermined position, for example, between two adjacent substrates 14.
The rotor 20 of a magnetized planar material made by a cutting and drawing process includes a cover 21 having a round upper wall 21a and an annular peripheral wall 21b, and thirty-two magnetic poles 22-1 to 22-32 annularly arranged inside the upper wall 21a and the peripheral wall 21b of the cover 21, with a constant angular distance between each. The magnetic poles 22-1 to 22-32 are disposed in contact with a corner section where the upper wall 21a and the peripheral wall 21b merge with each other.
The annularly arranged magnetic poles 22-1 to 22-32 are evenly disposed so as to have a constant angular distance of 11.25 degrees (360 degrees/32=11.25 degrees) from each other, and to have small spaces 23-1 to 23-32 between each other. In the rotor 20, the upper wall 21a is fixed at its center to the center shaft 11 by appropriate means, and is disposed to cover the stator 10, such that tip-end faces of the substrates 14-1 to 14-24 are disposed opposing the magnetic poles 22-1 to 22-32.
Generally, a three-phase brushless motor includes 3N coils in the stator, and 4N magnetic poles in the rotor (N represents an integer of 1 or greater).
The reinforcing plate 30 is made of a planar metallic material, for example, of steel being cut into a generally rectangular shape, provided with an aperture 31 at a predetermined position (refer to FIG. 6). The bearing case 13, passing through the aperture 31, is fixed to the reinforcing plate 30 by appropriate means.
When three phases of an alternating current are represented by U, V, and W, the phase-U current flows through the coils 15-1, 15-4, 15-7, . . . 15-22, the phase-V current flows through the coils 15-2, 15-5, 15-8, . . . 15-23, and the phase-W current flows through the coils 15-3, 15-6, 15-9, . . . 15-24.
Three position-detecting Hall elements 16 are provided, the elements corresponding to the phase-U, phase-V, and phase-W currents.
Following is a description of the operation of a known brushless motor arranged as described above.
The rotor 20 rotates around the stator 10 with three-phase alternating current supplied from a three-phase alternating current source.
A phase-U position-detecting Hall element 16-1 controls the switching timing of the phase-U current by detecting a magnetic flux change according to the position of each of the magnetic poles 22-1 to 22-32 included in the rotor 20, that is, a magnetic flux change arising from the approach of each of the spaces 23-1 to 23-32. A phase-V position-detecting Hall element 16-2 and a phase-W position-detecting Hall element 16-3 also control the switching timing of the phase-V current and the phase-W current, respectively.
The rotor 20 can be rotated at a predetermined stabilized speed by thus controlling the switching timing of the current in each of the phases U, V, and W.
Now, magnetic paths between the magnetic poles 22 of the rotor 20 and the substrates 14 of the stator 10 are described as follows.
In FIG. 7, most of the magnetic paths from the magnetic poles 22 to the substrates 14 function as effective magnetic paths 40, while the remainder of the magnetic paths from the magnetic poles 22 becomes ineffective magnetic paths 41 and 42. The ineffective magnetic paths 41 are formed between the magnetic poles 22 and the upper wall 21a of the cover 21. The ineffective magnetic paths 42 are formed between the magnetic poles 22 and the reinforcing plate 30.
The effective magnetic paths 40 contribute to generation of the brushless motor torque, while the ineffective magnetic paths 41 and 42 do not contribute to the rotation of the motor.
A problem of the known brushless motor is found in that portions of the magnetic paths generated from the magnetic poles 22 of the rotor 20 to the upper wall 21a and the reinforcing plate 30 are ineffective for the brushless motor torque, thereby decreasing the effective magnetic paths formed between the magnetic poles 22 and the substrates 14, resulting in an inefficient motor.